1. Field of the Invention
The present invention relates to base current helpers, and more particularly to base current helpers as are used in such circuits as current mirrors to help overcome the effects of base current in a collector circuit of the mirroring device.
2. Prior Art
Current mirrors are commonly used in many circuits to provide one or more currents equal to or proportional to a reference current for biasing as well as various other purposes. A commonly used current mirror is shown in FIG. 1. The current mirror shown consists of diode connected transistor Q1, the base and collector of which are connected to the base of transistor Q2. To the extent that the transistors are high gain transistors so that base currents can be ignored and transistors Q1 and Q2 are substantially identical, the collector current in transistor Q2 will equal the collector current in transistor Q1, namely the reference current I1. In fact, however, the base current for both transistors Q1 and Q2 flows in the collector circuit of transistor Q1, so that considering base currents, the mirrored current in the collector of transistor Q2 will be equal to I1-2IB, where I1 is the reference current in the collector circuit of transistor Q1 and IB is the base current in each of transistors Q1 and Q2.
To the extent the gain of the transistors is limited, the mirrored current will be in error. For instance, a simple current mirror of this type suffers a 10% error in a 1-to-1 current mirror with betas (beta is the ratio of collector current to base current of a transistor) of 20. Also, it is frequently desired to make transistor Q2 n times as large as transistor Q1 so that for the same base-emitter voltage as transistor Q1, transistor Q2 would conduct approximately n times the reference current I1. However, with limited beta transistors, the simple current mirror will not produce ratios significantly above 1-to-1 effectively. Also, current mirrors are frequently used to mirror a reference current on a 1-to-1 or other basis to a plurality of transistors rather than the single transistor Q2 of FIG. 1, increasing the error of the current mirror because of the increased number of base current components in the collector circuit of transistor Q1.
The foregoing base current induced errors are not limited to bipolar junction transistors, but are particularly severe in the case of lateral PNP bipolar junction transistors because of the finite beta of such devices. As for many present day processes, these lateral devices have betas that may fall into the single digits. To prevent accuracy problems in current sources, base current helpers are typically employed. These buffers absorb the excess base current at the cost of biasing the additional buffer. Unfortunately, the present state of the art biases these helpers in class A, meaning that the standing current in the helpers must exceed the worst case possible demands to keep the current sources alive. This can be several times greater than the nominal required, and can require in excess of 10% of the current source value.
A typical prior art current mirror with base current helper may be seen in FIG. 2. Here, the current for the bases of transistors Q3 and Q4 is set by transistor Q5 responding to the collector voltage of transistor Q3. Thus the current in the collector of transistor Q4 is equal to I2 minus the base current of transistor Q5. Because of the isolation of the base current of transistor Q4 from the collector circuit of transistor Q3, this circuit is much more tolerant to the use of a transistor to which the current is mirrored (Q4) which is n times larger than the mirroring transistor (Q3). However, even this mirror with helper, while improved, will produce errors in excess of 5% for a 10-to-1 current ratio, assuming the same exemplary betas of 20.
FIG. 3 is a circuit diagram for another prior art base current helper. The circuit of FIG. 3 has the advantage of using NPN devices for the feedback loop. Such devices typically have significantly higher betas, and even if they didn't, their currents can be set independent of the currents in the mirror devices Q13 and Q14, reducing the error when compared to the mirror with helper of FIG. 2. Current ratios in excess of 10-to-1 are possible with the circuit of FIG. 3 at accuracies at around 1%. In this circuit, the reference current I6 is fed to the collector of transistor Q13. Transistor Q16 acts to force the collector of transistor Q13 to bias at a potential relative to the base of transistor Q13 determined by the voltage V2. Thus, the base to collector voltage of transistor Q13 is forced to a known potential difference. When this occurs, the collector current through transistor Q13 roughly matches the reference current, and with the exception of Early voltage effects, the current through the collector of transistor Q14 will equal n times the reference current I6(transistor Q14 being n times as large as transistor Q13).
The NPN base current helper of FIG. 3 may be found in a December 1993 IEEE Journal of Solid-State Circuits, Vol. 28, No. 12, pp. 1246-1253. The right half of FIG. 8 on p. 1250 of the Journal corresponds to FIG. 3 of this disclosure. This circuit is the preferred existing method for building high ratio, high accuracy PNP current mirrors in processes with lateral PNPs. V2 can be made to be any reference, but the most common practices are to replace it with NPN diodes, Schottky diodes, or a ground referenced voltage to make the voltage across current source I6 supply-voltage independent. The major disadvantage to this solution (and all other prior art the inventor has found) lies in the fact that the structure composed of Q16 and whatever implementation of voltage V2 is used must be biased by fixed current source I8. This current source must be set at a level that is determined by the absolute worst case base current of Q13 and Q14, which varies dramatically over processing and temperature for most processes. The excess current required by the circuit of FIG. 3 is primarily the collector current of transistor Q16, which is basically wasted except for the fact that it keeps transistor Q16 active. As will be seen in the detailed description of the invention herein, the present invention comprises a class of circuits that can be employed to eliminate this wasted excess current while still assuring that transistor Q16 remains active.